Traffic

Frequently Asked Questions

What Is Traffic Engineering?

The content of this page is taken almost verbatim from a booklet called ''Traffic Engineering: What? Why? How?'', published by the Arizona Highway Department (now the Arizona Department of Transportation, Intermodal Transportation Division) in the early 1970s.

Despite this information being over 20 years old and the innumerable technological innovations in the field, the basic principles of traffic engineering have not changed: achieving the safe, efficient, and convenient movement of people and goods using streets, roads and highways.

Traffic engineering is that phase of engineering that deals with the planning, geometric design and traffic operations of roads, streets and highways and their networks, terminals, abutting lands and relationships with other modes of transportation for the achievement of safe, efficient and convenient movement of people and goods.

Traffic engineering applies engineering principles that help solve transportation problems by considering the psychology and habits of the transportation system users.

Many people still wonder why a traffic problem is so difficult that an engineer should be called upon for a solution. Why not just install a traffic signal, or raise/lower the speed limit, or erect more signs?

One of the greatest obstacles a professional traffic engineer faces in applying sound principles of traffic engineering is competing theories from vocal non-traffic engineers who lack expertise. The unfortunate result is the creation of traffic hazards when false theories are put into effect. Whenever unnecessary or excessive traffic controls are installed, hazardous traffic conditions usually result.

The role of traffic engineers may be compared to that of the medical profession in protecting the public. As trained professionals, traffic engineers look at the symptoms of general traffic conditions, and to make a competent diagnosis, they take traffic counts, analyze accident statistics, study speed data, examine roadway conditions, conduct research and study what other professionals are doing and the results they have achieved.

Just as the doctors' decisions are accepted in matters regarding health, even though the medicine may be bitter or the needle painful, so should the decisions of professional traffic engineers be given the prime consideration.

Experience has shown that safety goes hand in hand with smooth traffic operation. Disrupting the smooth flow of traffic increases the probability of accidents.

Erratic traffic operation may be caused by vehicles stopping or slowing in the roadway, passing and weaving maneuvers, or other surprise elements. For example, unwarranted traffic signals, unreasonably low speed limits and too many signs may cause driver confusion and indecision.

Slower speed does not necessarily mean safer traffic operation. The chances of a drivers becoming involved in an accident are lowest when they are traveling at the average speed of traffic.

"Uniformity" means treating similar situations in the same way. Applied to traffic engineering, uniformity simplifies the drivers' tasks because it aids in instant recognition and understanding. Uniformity aids police, courts and road users by giving everyone the same interpretation. It aids public-highway officials through economy in manufacturing, installing, maintaining and administering the roads.

The Manual on Uniform Traffic Control Devices is the publication that sets forth the basic principles that govern the design and usage of traffic control devices. The manual was prepared by a national committee that included state, county and municipal representation. The standards in this manual, with certain exceptions, apply to all streets and highways regardless of the government agency having jurisdiction.

Legal speed limits are established by law (in Arizona) and may be changed only when justified on the basis of an engineering study.

A widely accepted principle is to set speed limits as near as practicable to the speed below which 85 percent of the vehicles are traveling on the highway. Experience has shown that approximately 85 percent of motorists drive at a speed that is reasonable and prudent. Established speed limits therefore encourage voluntary compliance because they appear reasonable to the public. Those 15 percent of drivers who will not comply with reasonable speed limits are the drivers who are subject to enforcement action.

Posted speed limits have very little effect on actual traffic speeds. There is a common belief that the posting of speed limit signs will cause drivers to react accordingly. This is not true and is why posted speed limits must be realistic to achieve compliance.

Unrealistically low speed limits invite violation even by responsible drivers. Enforcement of unreasonably low limits sets up the so-called "speed trap," which results in poor public relations. The posting of proper speed limits has the beneficial effects of smoothing traffic flow and aiding effective law enforcement.

Traffic signals should be installed when they will alleviate more problems than they will create. This determination must be based on an engineering study.

A warranted traffic signal that is properly located and operated may provide for more orderly movement of traffic and may reduce the occurrence of certain types of accidents. On the other hand, an unwarranted traffic signal can result in increased delay, congestion and accidents.

Many people seem to believe that traffic signals are the answer to all traffic problems at intersections. If this were true, no traffic engineer would deny a request for a signal. However, a traffic signal only functions by stopping traffic, and any time a motor vehicle is stopped in the road, there is the potential for an. It does not matter whether the stop is caused by a flat tire, a left turn into a driveway, or a traffic signal; the possibility exists that a following motorist will not notice the stopped vehicle until it is too late.

What traveler has not experienced a traffic signal suddenly turning amber a few hundred feet away? Who has not experienced waiting in a long line of cars for a traffic signal to change, moving ahead a few feet, and then having the signal turn red again? To avoid these kinds of inconveniences and the increased potential for accidents, the need for traffic signals should be based on competent engineering study.

The principal purpose of guide signs is to direct travelers to their destinations by the best route. However, it is not feasible to install signs listing all the possible destinations that may be reached from the highway. Drivers must make reasonable preparation for locating their destinations and have information that is readily available on road maps or GPS systems.

Guide signs require simplicity and clarity because drivers of moving vehicles are unable to read lengthy or complicated messages on signs. For this reason, sign messages should not exceed three lines.

On freeways, high traffic speeds demand that the number of signs be limited to those absolutely essential for the guidance of the motorist. Freeway exits are identified by the exit number, the route number or the name of the intersecting road. Certain additional messages may be provided where justified.

In rural areas signs may be installed to direct travelers to services such as roadside rests, gas, food and lodging.